WASHINGTON -- The critical base of the ocean food web is shrinking as the world's seas warm, new NASA satellite data show. The discovery has scientists worried about how much food will grow in the future for the world's marine life.

The data show a significant link between warmer water _ either from the El Nino weather phenomenon or global warming _ and reduced production of phytoplankton of the world's oceans, according to a study in Thursday's journal Nature.

Phytoplankton are the microscopic plant life that zooplankton and other marine animals eat, essentially the grain crop of the world's oceans.

"Everything else up the food web is going to be impacted," said oceanographer Scott Doney of the Woods Hole Oceanographic Institute. "What's worrisome is that small changes that happen in the bottom of food web can have dramatic changes to certain species at higher spots on the food chain."

This is yet another recent scientific study with real-time data showing the much predicted harmful effects of global warming are not just coming, but in some cases are already here and can be tallied scientifically, researchers said.

A satellite commissioned by NASA tracked water temperature and the production of phytoplankton from 1997 to 2006, finding that for most of the world's oceans when one went up the other went down and vice versa, said study lead author Michael Behrenfeld, a biological oceanographer at Oregon State University.

As water temperatures increased from 1999 to 2004, the crop of phytoplankton dropped significantly, about 200 million tons a year. On average about 50 billion tons of phytoplankton are produced yearly, Behrenfeld said.

During that time, some ocean regions, especially around the equator in the Pacific, saw as much as a 50 percent drop in phytoplankton production, he said.

However, the satellite first started taking measurements in 1997 when water temperatures were at their warmest due to El Nino. That's the regular cyclical warming of part of the Pacific Ocean that affects climate worldwide.

After that year, the ocean significantly cooled until 1999 and the phytoplankton crop soared by 2 billion tons during those two years.

"The results are showing this very tight coupling between production and climate," Behrenfeld said.

Phytoplankton, which turn sunlight into food, need nutrients such as nitrogen, phosphates and iron from colder water below, Behrenfeld said. With warmer surface water, it's harder for the phytoplankton to get those nutrients.

Behrenfeld said the link between the El Nino changes and phytoplankton production is clear. For years scientists warning about climate change have said warmer waters will reduce phytoplankton production and this shows it's happening, he said.

Other oceanographers agree with the El Nino link but said with only a decade of data it is harder to make global warming connections.

"It's something you certainly can't ignore, because its potential is quite significant," said James Yoder of the Woods Hole Institute. "But there are some caveats because of the shortness of the record."

Another worry is that with reduced phytoplankton, the world's oceans will suck up less carbon dioxide, increasing the Earth's chief global warming gas, said NASA ocean biology project manager Paula Bontempi. That's because phytoplankton take carbon dioxide out of the atmosphere in making food.

This is at least the third significant peer-reviewed research paper in the past six months showing that long-anticipated global warming biological side effects are already happening.

A study earlier this year linked increases in Western U.S. wildfires to global warming and a mega-study showed that dozens of species of plants and animals were dying off from global warming.

"What you're looking at is almost an avalanche of each individual effect," said Stanford University biological sciences professor Stephen Schneider. "As it gets warmer and as we measure more things, the evidence accumulates."